Crafty : JavaScript Game Engine, HTML5 Game Engine
React : A JavaScript library for building user interfaces
Slate : Slate is a completely customizable framework for building rich text editors.
Slate lets you build rich, intuitive editors like those in Medium, Dropbo...
stream.js : stream.js is a tiny stand-alone Javascript library that unlocks a new data structure for you: streams.
swfobject : SWFObject 2 offers two optimized Flash Player embed methods; a markup based approach and a method that relies on JavaScript & offers a JavaScript API ...
EnhanceJS is a new Javascript framework designed to improve the application of Progressive Enhancement by first testing browser capabilities for key Javascript and CSS support before applying advanced styles and scripts to the page.